Norris received massage treatment Wednesday with Houston's training staff and said that team trainers are "monitoring the old back" closely.

Norris had shut down the Royals through six innings, allowing five hits and no runs before coming back out for his seventh-inning warm-ups.

But a long Astros turn at the plate in the bottom of the sixth left him tight, and Astros manager Bo Porter noticed that Norris wasn't finishing his warmups.

"If you let Bud tell it, he probably could have went on, but I made the decision immediately when he said it was tight," Porter said. "I'm more concerned with us getting these things right so it doesn't linger."

Norris, 28, said he has never dealt with back issues before, so the injury is "completely new territory" for him.

"It's just making sure you're stretched and loose so that your body isn't cold when you get out there so you don't pull something," he said. "I don't think that was the case [Tuesday]."

Norris' strong outing prior to re-aggravating the injury -- which also took him out of his May 14 start in Detroit -- put his ERA at 1.93 in six home starts this season.

His next start would likely be scheduled for Monday at home against the Rockies.